The Science
Tropical forests vary widely in woody productivity, tree mortality, and biomass carbon stocks, even for forests of the same age. Reviewing previous studies, we find that productivity is highest in warm, wet forests on fertile soils, whereas mortality is higher at higher soil fertility and higher disturbance. This in turn means that biomass is higher at higher rainfall and temperature, lower disturbance, and intermediate soil fertility.

The Impact
A mechanistic understanding of tropical forest biomass, productivity, and mortality is critical to accurately representing these processes in global vegetation models and predicting responses to global change. By reviewing empirical studies, we describe the state of current knowledge regarding the mechanisms driving carbon dynamics in tropical forests and highlight areas requiring further study.

Summary
We searched the literature for studies of among-site variation in our focal variables (biomass, productivity, and woody residence time) in mature, unlogged tropical forests, or in secondary forests when controlling for stand age.  Woody productivity and biomass decrease from wet to dry forests and with elevation.  Within lowland forests, productivity and biomass increase with temperature in wet forests, but decrease with temperature where water becomes limiting. Woody productivity increases with soil fertility, whereas residence time decreases, and biomass responses are variable, consistent with an overall unimodal relationship. Areas with higher disturbance rates and intensities have lower woody residence time and biomass.  These environmental gradients all involve both direct effects of changing environments on forest carbon fluxes and shifts in functional composition – including changing abundances of lianas — that substantially mitigate or exacerbate direct effects. Biogeographic realms differ significantly and importantly in productivity and biomass even after controlling for climate and biogeochemistry, further demonstrating the importance of plant species composition. Capturing these patterns in global vegetation models requires better mechanistic representation of water and nutrient limitation, plant compositional shifts, and tree mortality.

Contact: Helene C. Muller-Landau, Staff Scientist, Smithsonian Tropical Research Institute, MullerH@si.edu

Funding
KCC was supported as part of the Next Generation Ecosystem Experiments-Tropics, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research.

Publications
Muller-Landau, H.C., Cushman, K.C., Arroyo, E.E., Martinez Cano, I., Anderson-Teixeira, K.J., and B. Backiel (2020). Patterns and mechanisms of spatial variation in tropical forest productivity, woody residence time, and biomass. New Phytologist,  https://doi.org/10.1111/nph.17084.

A logging module added in FATES allows the impacts of forest degradation be represented in Earth system models.

The Science
A team of scientist in NGEE-Tropics developed a selective logging module for the Functionally Assembled Terrestrial Ecosystem Simulator (FATES), a new generation ecosystem demography model for representing vegetation dynamics in Earth system models. This module represents logging practices that span the range from complete clear-cuts to highly selective logging treatments practices. Benchmarking of the module against selective logging experiments in the Tapajós National Forest in Brazil showed that the model is able to capture important effects of logging on forest structure and carbon, water, and energy fluxes.

The Impact
Tropical forest degradation not only alters carbon stocks and carbon fluxes, but also impacts physical land surface properties. Such impacts are poorly quantified to date due to difficulties in accessing and maintaining observational infrastructures, as well as the lack of proper modeling tools. The new module creates a foundation for future model developments to include a wider set of land use transitions and regionally- and temporally-resolved variation in logging practices to better simulate the effects of forest degradation in the Earth system.

Summary
The module is designed to mimic the ecological, biophysical, and biogeochemical processes at a landscape level following a logging event. This is achieved by specifying the timing and aerial extent of logging events, splitting the logged forest patch into disturbed and intact patches, determining the survivorship of cohorts in the disturbed patch, and modifying the biomass and necromass pools following logging. The logging module is parameterized to reproduce a selective logging experiment at the Tapajós National Forest in Brazil and benchmarked against available field measurements.  Our results suggest that the model permits the coexistence of early and late successional functional types following disturbance. The model also realistically characterizes the seasonality of water and carbon fluxes and stocks, the forest structure and composition, and the ecosystem succession after disturbance.

Contacts (BER PM): Daniel Stover, Terrestrial Ecosystem Science, Daniel.Stover@science.doe.gov
PI Contact: Jeff Chambers, Lawrence Berkeley National Laboratory, jchambers@lbl.gov
Ruby Leung, Pacific Northwest National Laboratory, Ruby.leung@pnnl.gov

Funding
This research has been supported by the U.S. Department of Energy, Office of Science (grant no. 66705), the São Paulo State Research Foundation (grant no. 2015/07227-6), the National Aeronautics and Space Administration (grant no. 80NM0018D004), and the National Science Foundation (grant no. 1458021).

Publications
Huang, M., Xu, Y., Longo, M., Keller, M., Knox, R. G., Koven, C. D., and Fisher, R. A.: Assessing impacts of selective logging on water, energy, and carbon budgets and ecosystem dynamics in Amazon forests using the Functionally Assembled Terrestrial Ecosystem Simulator, Biogeosciences, 17, 4999–5023, https://doi.org/10.5194/bg-17-4999-2020, 2020.

The Science
Soil structure and topographic features explain within-species and local-scale variability in tree growth. Using percolation theory, it is shown that root growth into the surface soil layers (0–2 m) tends to follow paths with minima in resistance, which in turn maximizes water flow and nutrient delivery rates that regulate growth and transpiration rates.

The Impact
This analysis generates the hypothesis that within-species and local-scale variability in transpiration, and hence tree growth, can be reliably predicted by soil structure because root growth into the surface soil layer (0–2 m) tends to follow paths with minima in resistance, which in turn maximizes water flow and nutrient delivery rates. It is shown that the variability in tree height may be accounted for by a single analytical equation for the growth of plants, which is governed by the optimal root length within the radial extent of the roots in the surface soil layer.

Summary
It is well established that tree growth rates are proportional to transpiration rates. This study investigates whether variability in tree growth on local scales and within species is related to constraints placed on transpiration by soil structure and other environmental conditions and not by constraints related to available soil water. Percolation theory is applied to explain how root-soil interactions regulate transpiration where root growth into the surface soil layer (0–2 m) tends to follow paths with minima in resistance, which in turn maximizes water flow and nutrient delivery rates that regulate growth. It is shown that the variability in tree height may be accounted for by a single analytical equation for the growth of plants, which is governed by the optimal root length within the radial extent of the roots in the surface soil layer. The scientific results are based on testing several scenarios of different topographic features (curvature, slope, and aspect), soil characteristics, and climate ranges. The results can be used for analytical calculations of NPP within the scope of the NGEE-Tropics project.

Contacts (BER PM): Daniel Stover, SC-23.1, Daniel.Stover@science.doe.gov (301-903-0289)

PI Contact: Boris Faybishenko, Lawrence Berkeley National Laboratory, bafaybishenko@lbl.gov

Funding
This research was partially supported by the NGEE-Tropics and DEDUCE projects, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, and Office of Advanced Scientific Computing under contract DE-AC02–05CH11231.

Publications
Hunt AG, Faybishenko B, Powell TL (2020) A new phenomenological model to describe root-soil Interactions based on percolation theory. Ecological Modelling. doi: 10.1016/j.ecolmodel.2020.109205.

The Science
The amount of water that tropical trees lost from their stems during drought conditions, when trees lack access to soil water, was correlated with their bark water vapor conductance, i.e., the leakiness of bark to water vapor. This suggests that water loss through bark may be an important and overlooked mechanism that influences stem dehydration and drought performance.

The Impact
Predicting drought performance among trees is a major challenge. These results suggest that incorporating the rate of water loss from bark can help to predict the rate at which trees dehydrate and die during droughts.

Summary
Saplings of several tree species in Panama were measured for stem water content during well-watered conditions and drought conditions in forest understories and in a shadehouse experiment to assess stem water deficit during drought. Saplings of the same species were collected and measured for bark water vapor conductance. In both datasets, bark water vapor conductance was correlated with stem water deficit among species that lacked assess to soil water.

Contacts (BER PM): Daniel Stover, SC-23.1, Daniel.Stover@science.doe.gov (301-903-0289)

PI Contact: Brett Wolfe, School of Renewable Natural Resources, Louisiana State University Agricultural Center, Baton Rouge, LA, Wolfe1@lsu.edu

Funding
This project was supported by the Next Generation Ecosystem Experiments-Tropics and by the US Department of Energy, Office of Science, Office of Biological and Environmental Research.

Publications
Wolfe BT. 2020 Bark water vapour conductance is associated with drought performance in tropical trees. Biol. Lett. 16: 20200263. http://dx.doi.org/10.1098/rsbl.2020.0263

The Science
Terrestrial ecosystem dynamics, including carbon stocks and biosphere-atmosphere fluxes of carbon dioxide (CO2), water (H2O), and volatile organic compounds (VOCs) are dramatically changing in response to climate factors such as trends in surface warming and a higher frequency and intensity of large-scale droughts and associated insect infestation epidemics. Understanding the mechanisms that drive forest responses to climate change is vital for predicting how the structure and function of natural and managed ecosystems will react to environmental change including alterations in carbon and H2O cycling, and the ecosystem services and products provided. Therefore, understanding the underlying biochemical, physiological, and ecological processes including plant growth and development, abiotic and biotic stress responses, mortality, and ecological succession and forest recovery are critical for accurately predicting the future of forest structure and function.

The Impact
We highlight cell wall-derived emissions of methanol and acetic acid, along with associated changes in cell wall structure and function as a common thread among the processes that underlie ecosystem responses to climate change. Thus, interdisciplinary studies linking cell wall biochemistry and metabolism with plant physiology and biosphere-atmosphere gas exchange will lead to better predictive understanding of the mechanisms through which cell wall esters facilitate forest response to climate extremes. Of particular interest are high latitude forests responding to rapid warming through expansion of deciduous broadleaf trees and commensurate declines in evergreen conifer trees. These distinct plant functional types vary in their leaf phenological cycles and cell wall composition, with deciduous trees undergoing seasonal leaf emergence and senescence while conifer trees retain their needles over the winter months. This may impact the timing, spatial distribution, and magnitude of biosphere-atmosphere fluxes of VOCs, CO2, and H2O in such changing forests in the future.

Summary
While little is known about the functions of cell wall ester modifications in trees, evidence from model plant systems like Arabidopsis thaliana suggests that they may be highly dynamic, playing central roles in cell growth, tissue development and function, participating in sensing and signaling pathways involved in cell wall remodeling in response to stress, and integrate into primary C1-3 metabolism. Although the reservoirs and fluxes of carbon through acetylated and methylated cell wall polysaccharides are potentially large, they remain poorly characterized. Methanol and acetic acid products of cell wall de-esterification may integrate whole plant metabolism by being transported over large distances within the plant via the transpiration stream and extracellular air space and enter central metabolism in distant tissues, or be emitted into the atmosphere as gases. Moreover, cell wall-derived methanol and acetic acid may be intimately involved in signaling and immune responses as an essential component of environmental plant monitoring systems; changes in cell wall esters during stress induce signaling via damage-associated molecular patterns, which in turn activate immunity responses. This link is mediated by the role of cell wall esters in central metabolism and plant sensing and signaling pathways involved in cell wall structural remodeling and “green immunity”.

Contact: Kolby Jardine, Lawrence Berkeley National Lab, kjjardine@lbl.gov

Funding
This material is based upon work supported by the U.S. Department of Energy (DOE), Office of Science, Office of Biological and Environmental Research (BER), Biological System Science Division (BSSD), Early Career Research Program (FY18 DOE National Laboratory Announcement Number: LAB 17-1761), Topic: Plant Systems for the Production of Biofuels and Bioproducts, under Award number FP00007421 to Lawrence Berkeley National Laboratory.

Publications
R.A. Dewhirst, J.C. Mortimer, K.J. Jardine, (2020) Do cell wall esters facilitate forest response to climate?, Trends in Plant Science.

The Science
As an interplay of chronic drivers and transient disturbances are introduced to forests, dynamic changes occur in the reproduction, growth, and mortality of trees. Combining analyses from multiple studies, researchers led by Pacific Northwest National Laboratory investigated the demographic drivers of forest dynamics, the disturbances that drive them, and the pervasiveness and impact of these effects worldwide.

The Impact
Ongoing changes in both environmental drivers and disturbance regimes appear to be consistently increasing mortality and forcing forests towards shorter and younger stands. This reduces potential carbon storage and impacts the biodiversity and, ultimately, the surrounding climate. The pervasive shifts in forest vegetation dynamics are already occurring and are likely to accelerate under future global changes, with consequences for climate forcing.

Summary
In a collaborative study, more than twenty researchers investigated the influence of today’s climate conditions on the dynamics and stature of forests. Combining data and observations from more than 160 previous studies worldwide, the research team found that tree stands are losing their potential for obtaining height and enduring lifespans. Researchers identified consistent mortality trends resulting from specific chronically changing drivers—rising temperature, increasing CO2 levels, and transient disturbances including wildfire, drought, and land-use change. These factors have thrown out of balance three important characteristics of a diverse and thriving forest: (1) recruitment, which is the addition of new seedlings to a community; (2) growth, the net increase in biomass or carbon; and (3) mortality, the loss of a plant’s ability to reproduce. As a result, forest vegetation and canopies decline and overall recovery is lessened, resulting in ecosystems dominated by novel species that have replaced the original community. Since the resulting forest loss comes from both natural and land-use change drivers, all of which are predicted to increase in magnitude in the future, it is highly likely that tree mortality rates will continue to increase while recruitment and growth will respond to changing drivers in a spatially and temporally variable manner. The net impact will be a reduction in forest coverage and biomass, with mixed effects on biodiversity. This study forms the basis for investigations regarding the patterns and processes underlying the shifts in forest dynamics, all of which can be tested using emerging terrestrial and satellite-based observation networks.

Contacts (BER PMs): Dan Stover, Program Manager, Terrestrial Ecosystems Science, Dan.Stover@science.doe.gov

PI Contact: Nate McDowell, Pacific Northwest National Laboratory, Nate.McDowell@pnnl.gov

Funding
This research was supported by the Office of Science of the U.S. Department of Energy (DOE) as part of the Terrestrial Ecosystem Science (TES) Program and the Next Generation Ecosystem Experiment-Tropics (NGEE-Tropics) project. The Pacific Northwest National Laboratory is operated for DOE by Battelle Memorial Institute under contract DE-AC05-76RL01830.

Publication
McDowell, NG, et al. 2020. “Pervasive shifts in forest dynamics in a changing world.” Science 368(6494):eaaz9463. https://doi.org/10.1126/science.aaz9463

Related Links
https://science.sciencemag.org/content/368/6494/eaaz9463